This invention relates to a field plate structure for high voltage semiconductor devices, and more specifically relates to a combined field plate structure which minimizes the curvature of the electric field in a semiconductor body in regions below the rim of the field plate.
It is well known that relatively high electric field stress is produced in the body of a semiconductor device if the main electrode of the device is sharply terminated. This is because the electric field lines curve sharply at regions underlying the outer periphery of the field plate, and limit the ability of the device to withstand reverse voltage.
The curvature of the electric field lines has been controlled in the past by guard rings which encircle the outer periphery of the device and by field plates which are extensions of the device electrodes, but are spaced from the semiconductor substrate as by overlying a silicon dioxide layer on the semiconductor surface. This will then reduce the curvature of the electric field lines beneath the contact area of the electrode, although the electric field lines still can bend fairly sharply beneath the outermost periphery of the stepped field plate. To reduce this effect, it is also known to provide a second step in the field plate, but this requires the formation of a step in the oxide which supports the field plate and defines its shape. The provision of a step in the oxide requires additional complex processing steps.